Austenitic alloy



July 9, 1963 w. w. DYRKAcz ETAL 3,097,092

AUSTENITIC ALLOY (Sawww) am |001.

INVENTORS WasilW. Dyrkucz, Clifford R. Hostings und Richard K, PilerATTORNEY July 9, 1963 w. w. DYRKAcz r-:rAL 3,097,092

AUSTENITIC ALLOY 2 Sheets-Sheet 2 Filed July 5, 1961 mm. mm.

ION

(Sawulw) am w01 INVENTORS Wusil W. Dyrkacz, Clifford R Has'tings RichardK. Pfler BY W ATTORNEY and nited States Patent 3,097,092 AUSTENI'IICALLOY WaslLW. Dyrlracz, Niskayuna, and Cliiiord R. Hastings,

'I.`roy, N.Y.-, and Richard K. Pitler, Gihsonia, Pa., assiguors`toAllegheny Ludlum Steel Corporation, Brackewliifidge,V Pa., acorporation, of Pennsylvania Filed July s, 196i, ser. No. 121,886 13Claims. (Cl. 75-128) This invention relates to stable, austenitic,age-hardenable alloys suitable for use at temperatures of up to about1600 F., and in particular to stable, austenitic, agehardenable alloyswhich are especially suitable for use as-valves and valve components inexhaust systems of inter-nal combustion engines.

The development of new metals and alloys for use as valves and valvecomponents which exhibit 11i-gh temper-attire Vproperties superior tothose metals and alloys which are commercially available today, haslagged the pace of other similar metallurgical developments becausecompetitive considerations have emphasized valve cost reductionl Withoutthe sacrice of physical, chemical and mechanical properties. Thus thesupplier who manufactures the metals and alloys from which such valvesare made must oler a product which possesses the requisite mechanicalproperties .such as stress-rupture strength and hardness, chemicalproperties such as corrosion and oxidation resistance, and physicalproperties such as resistance tothermal fatigue, hot stretching andthermal shock, and atthe same .time the suppliers product must alsoexhibit the abil-ity ,to be fabricated at a minimum cost. Theseconsiderations oef cost have had their manifestations portrayed thechemical composition of the metal, the hot workability o f the metal,and the machinability of the meta-l. Thus, with respect to the costfactor as it relates to the chemical composition, it is incumbent uponthe metal manufacturer to design his compositionY so that the costsofrawmaterial alloying components are minimized without the-sacrifice of therequisite mechanical, chemical andiphysical properties necessary in thefinished product. In addition, the chemical composition must also bebalanced so that the metal will exhibit excellent hot workability,thereby reducing to a minimum all scrap losses which would be incurredduring the hot working of the metal from the cast ingotinto thesemi-finished mill productfrom which the finished product Vismanufactured. In the` case of-valvesand valve components, thesemi-iinished mill product is usually bar stock fwhich must exhibit goodnpsettability .and extrudability over .a -wide temperature rangeinto'anished valve. Moreover, the semi-finished mill product which themetal manufacturer supplies'must also exhibit excellent machinability,especially in the case of valves and valve components, in order tominimize thecost of .the iinished product. The machinability exhibitedby,the ymetal or alloy is integrally related to the chemical com-positionof the metal or alloy; therefore, the balanced relationship of thealloying components is most critical.

One of the commercially available valve compositions inrpresent usetoday is an austenitic alloy, referred to hereinafter as Alloy X,whichrpossesses a nominal commercial analysis of about .55% carbon,about 9.5% manganese, low silicon, about 21% chromium, about 4% nickeland about `0.40% nitrogen, about 0.06% sulfur and thejbalance iron. Thiscomposition possesses an acceptable combination` of stress-rupturestrength, hardness, corrosion and oxidation resistance, resistance tothermal fatigue, hot stretching and thermal shock. However, the designerof the alloy found it to be essential to include about v0.016% sulfur inthe composition in order for the alloy-to .exhibit an economical degreeof maehinability. However, the machinability was imparted to the alloyat 3,097,092 Patented July 9, 1963 the sacriiice of hot Workability, andwhere the sulfur content exceeded about 0.07% sulfur, high scrap lossesresulted in the commercial mill processing of this material. Inaddition, the over-all effect of the alloying components was such thatan extremely narrow range of the sum of the carbon-l-nitrogen content(0.92% to 0.98%) had to be maintained. Where the sum of thecarbon-|-nitrogen was below this critical ran-ge, the iinished, heattreated product did not exhibit the requisite hardness. On the otherhand, where the sum of the carbon-l-nitrogen exceeds the upper limit,considerable difficulty is encountered in shearing and machiningoperations. In addition, where the sum of the carbon-i-nitrogen exceedsabout 0.98%, the alloy is diicult to work in the blooming mill, it beingfound that a greater number of passes and larger power requirements arenecessary, thus adding to the cost of fabricating the alloy. While ahigher temperature would rectify the ditiiculties encountered in theblooming operation, such higher temperature cannot be employed wheremore than about 0.06% sulfur is present because the alloy becomeshotshort.

Another composition which has found some acceptance in commercial usagetoday is an alloy referred to herein as Alloy Y and which contains a lowcarbon content, about 5% nickel, about 5% manganese and about 21%chromium. Criticism of this material has resulted from the fact that thematerial could not be utilized in manufacturing a one-piece valve.Apparently the carbon content is insuicient for developing the requisitehardness. Therefore, in order to maintain stem and tip wear resistancein valves, it has been found preferable to make the stem and tip'of thevalve out of a different material.

In order to 'alleviate these conditions, the alloy of the presentinvention possesses mechanical properties akin to those exhibited byAlloy X, yet the alloy of the present invention exhibits excellent hotworkability and machinability Without the necessity of utilizing highsulfur contents, thus obviating any hot workability problems which havebeen associated with the prior art alloy.

An object of the present invention is to provide an alloy which issuitable tfor use at elevated temperatures of up to about 1600" F. andwhich possesses an acceptable combination of mechanical, chemical'andphysical properties which make said alloy suitable for use as valves andvalve components.

Another object of this invention is to provide a stable, austenitic,age-hardenable alloy suitable for use as valves and valve componentsoperating at `elevated temperatures of up to about 1600" F., which alloyis characterized by having a balanced chemical composition within narrowlimits.

A further object of the present invention is to provide a stable,austenitic, age-hardenable alloy which is suitable for valves and valvecomponents operating at temperatures of up to about l600 F. and whichpossess excellent hot workability and machinability through a balancedchemical composition.

A lmore specific object of the present invention is to provide a stable,austenitic, age-hardenable alloy having a critical balance between thecarbon, nitrogen', manganese, nickel, chromium and silicon contents,together with a very narrow range of sulfur content and which possessesexcellent hot workability and machinability.

A further speciiic object of the present invention is to provide astable, austenitic, age-hardenable alloy suitable for -use as valves andvalve components suitable for use at temperatures of up to about 1600 F.and which has a .critically balanced composition between the austeniteformers carbon, nitrogen, manganese and nickel, and the `ferriteforrnerschromium and silicon and which exhibits excellent hot lworkability andmachinability without the -use of excessive amounts of sulfur or otherexpensive machinability-improving elements.

Other objects of this invention will become apparent when taken inconjunction with the drawings in which:

FIGURE 1 is a graphic illustration of the relative machinability ofalloys of both the present invention and prior art alloys, each in thesolution heat treated condition, and

FIG. 2 is a graphic illustration of the relative machinability of thealloys of the present invention as well as prior art alloys inthesolution treated plus aged condition.

In its broader aspects, the alloy of the present invention contemplatesa composition which includes between 0.65% and 0.75 carbon, 5.50% and6.90% manganese, 0.45% and 0.85% silicon, 20.50% and 22.0% chromium,1.40% to 1.90% nickel, 0.18% to 0.28% nitrogen, from 0.025% to 0.055%sulfur and the balance iron with incidental impurities. Reference may behad to Table I which illustrates the broad range and the preferred rangeof composition of the alloy which cornes Within the scope of the claimswhich are appended hereto.

TABLE I Chemical Composition [Percent by weight] Element General OptimumRange Range 0. 65-0. 75 0. 68-0. 73 30 5. 50-6. 90 6. 00-6. 50 0. 15-0.85 0. 60-0. 70 20 50-22.00 21.00-21.50 l. 40-1. 90 l. 60-1. 75 0. 18-0.28 0. 20-0. 25 0 025-0. 055 0. G25-0.040

B Bal.

Each of the foregoing elements set forth in Table I performs a vitalfunction in the alloy of the present invention. Carbon is elfective forforming austenite which must be controlled within relatively narrowlimits in order to insure suflicient hot workability without adverselyaffecting the machinability of the alloy. In addition, carbon materiallycontributes to the hardness and strength of the alloy, and it has beenfound that at least 0.65% carbon is necessary in order to obtain therequisite strength, hardness and a sufficient amount of austenite withinthe microstructure of the alloy in order to obtain the optimumcornbination of hot workability and machinability. Carbon contents inexcess of about 0.75% adversely affect the machinability and contributeto reducing the strength and ductility of the alloy. Optimum resultsappear to be obtained when the carbon content is maintained within therange between about 0.68% and 0.73%. Manganese materially contributes tothe austenitic stability of the alloy, and at least 5.50% is necessary.Manganese contents in excess of about 6.9%, while being effective forincreasing the austenitic stability, adversely affect the machinabilityof the alloy because of the higher work-hardening rate imparted by thiselement. Optimum results appear to be obtained where the manganesecontent is maintained within the range between 6.00% and 6.50%. Siliconis present within the alloy, and within the range specified materiallycontributes to the oxidation and corrosion resistance of the alloy. Atleast 0.45% silicon has been found to be necessary, and silicon contentsin excess of about 0.85% appear to adversely affect the mechanicalproperties of the alloy, especially the stress rupture strength. Siliconis a ferrite-forming element, and as such must be critically balancedagainst the austenite-forming components as will be more fully explainedhereinafter. Optimum results are obtained when the silicon content ismaintained within the range between 0.60% and 0.70%.

Chromium is present within the range between 20.50% and 22.0%. Withinthis range the alloy possesses the required degree of corrosionresistance, especially in atmospheres containing combustion products ofleaded fuels. In addition, at least 20.50% chromium is necessary inorder to afford the alloy an acceptable measure of resistance tooxidation at elevated temperatures of about 1600 F., and particularly1100 F. to 1600 F. Chromium contents below about 20.50% result in rapidscaling at hot working temperatures, thereby adversely affecting dielife, and thus increasing manufacturing costs. Chromium is also aferrite former, and as such must be balanced against the austeniteformers to insure the proper degree of hot workability without adverselyaffecting machinability. The most satisfactory combination of propertiesoccurs when the chromium content is within the range between 21.0% and21.5%. At least 1.40% nickel is used within the alloy of the presentinvention in that it contributes to the hot strength and hardness of thealloy. However, the nickel content must be maintained at no greater than1.90%, since increasing the nickel content beyond about 1.90% adverselyaffects the hardness, the stress rupture strength and the ductility ofthe alloy.

Nitrogen is highly critical and is present within the alloy as aninterstitial hardener. Thus, it has a great influence on the strainhardening rate of the alloy. In addition, nitrogen is a potentaustenite-forming element and materially contributes to the strength andstability of the alloy. At least 0.18% nitrogen is necessary within thealloy in order to insure the proper strength and austenite stability.Increasing the nitrogen content to above about 0.28% results in a greatincrease in the strain hardening rate, adversely affecting themachinability of this alloy, and also increases the resistance todeformation at hot working temperatures, thereby materially increasingfabricating costs. Optimum resul-ts appear to be obtained when thenitrogen content is maintained within the range of about 0.20% and0.25%.

The present yalloy also contemplates the presence of sulfur within thecomposition. At least 0.025% has been found necessary in order to insurethe proper degree of machinability within .the alloy. Increasing thesulfur content to above about 0.055% does not cause any significantimprovement in the machinability, and in addition, sulfur contents inexcess of 0.055 seriously limit the hot workability of the alloy, thuscausing considerable diiiiculty in manufacturing the alloy into thesemi-finished mill product form. The balance of the alloy comprisessubstantially all iron, with incidental impurities normally found in themaking of such alloys.

The alloy of the present invention is conveniently made in the regularsteel mill manner. Substantial success has been realized `by making saidalloy utilizing the carbon electrode electric arc melting practice toobtain the heat of the desired chemical composition, following which themelt is cast into ingots which are thereafter hot rolled to a convenientsemi-finished mill product form, for example, bars. It should be pointedout, however, that other melting practices can be used with equalsuccess, and because of the critically balanced nature of the alloyingelements, to be referred to more fully hereinafter, the alloy of thepresent invention exhibits outstanding hot workability and may be hotworked in any suitable manner on any suitable equipment `and is notnecessarily lim.- ited to hot rolling.

The alloy of the present invention may be used in vanious forms, and, inthis respect, it has ibeen found that the alloy of the present inventionis responsive to various heat treatments to obtain a number olf desiredcombinations of properties. In particular, it has `been found that thealloy of the present invention may be solution Iheat treated at atemperature in the range between l800 F. and 2200 F. foi a .time periodranging between about 15 mintntes and about four hours, followed by aquench. In practice, it has been found that the use of a water quench isquite effective. Subsequently the `alloy may be aged at a temperaturewithin the range of about l200 F. to

about 1700 F. for a time period ranging between about one hour and about24 hours` and thereafter the alloy may be air cooled. When heat treatedwithin the broad temperature range set forth hereinabove, the alloy ofthe present invention exhibits a good combination `of creep rnpltureproperties, hardness and ductility without any adverse effect on themachinability or the corrosion resistance of the alloy.

Where the alloy is to be used in service which requires the highestcombination of creep rupture properties and a hardness of Rc 34 minimum,the Aalloy may be advantageously heat treated by subjecting it to asolution heat treatment at a temperature within the range between 11950F. and Z200 F. for a time period ranging between 15 minutes and twohours, followed by a rapid cooling-to room temperature. Thereafter, the`alloy may be yaged at a temperature Within the range between about 1350F. `and `about l500 F. for `a time period ranging between four hours andvabout 16 hours, followed by air cooling. When thus heat treated, inaddition -to the -alloys exhibiting the highest combination of creeprupture properties and a minimum hardness of 34 Re, no `adverse effectshave been noted in the machinability of the alloy or in 4the corrosionresistance of the alloy.

Some value manufacturers prefer to produce a valve which exhibitsintermediate creep rupture properties and which may be produced at `alow cost without any sacrifice inthe corrosion or oxidation resistancenor in the machinability of the alloy. This may be achieved by hotworking the alloy into the shape of a valve, solution heat treating thesemi-lnished valve at a temperature within the range between1950 F. and20150 F., lfollowed -by a rap-id cooling, :and thereafter machining thevalve shape Where necessary Ito its finish dimensions. rIlhe Valve inthe solution treated .condition may then be used directly in an internalcombustion engine where it will become aged in use Where the valveoperates at a temperature within the range between 1100 F. and 1600" F.

Where -it is desirable to manufacture the lowest cost valve whichpossess-es adequate creep rupture properties, resistance to oxidationand 'corrosion and without any adverse effect on the machlinability, thealloy may be hot worked, for example, by extruding or forging andupsetting, into the lshape of a valve and thereafter the seminishedvalve may be machined to the iinished product. As so fabricated, thevalve may beused directly in the engine, thus providing the lowest costvalve with adequate properties. Where substantially higher hardnessesare required, the alloy of the present invention is particularlyattractive because of the Vsimplified lheat treatment which is involved.Once again, the yalloy may be either forged or extruded into the shapeof a valve, and thereafter the alloy .is subjected to a heat treatmentat la temperature within the rangebetween 14'00" F. to 1700 F. `for atime period of up lto about eight `hours `and thereafter the alloy aircooled. As thus processed, the valve which is formed from `the alloyofthe present invention exhibit a hardness lof 38 Rc minimum. hardnessis substantially higher than the maximum hardness exhibited by theone-piece austenitic valves in substantially all internal combustionengines which are utilized in passenger cars manufactured today.

When the alloy `of the present invention is utilized for valves andvalve components, itis desirable that the allo-y from which such valvesand valve components are made [possess mechanical properties which areequivalent to or better than the mechanical properties exhibited by thecommercially available alloys in use today. Reference is respectfullydirected to Table II which contains a tabulation 'of the chemicalcomposition of -a number of alloys which weremade, some of which fallwithin the scope of the` alloy of thepresent invention, and some ofwhichfall outside the scope lof the present invention.

6. TABLE II` Chemical Composition [Percent byweght] Heat No. C Mn Si OrNi N vS Fe 69 6. 8 .63 20. 5- 1.9 .69V 6.6 .72.20.63: 1.9.' .51 6.3 .2621.7 l 2.0 l 68 6. 3 58 22. 0 2. 1 .56y 6.4 .32 22.0 2.2` 61 6. 4 1120.8 2.4 .76 6.9 .23' 20.6'v ,2.3 .70 6.2 .83 20:5A 1.9 Y .51 9.5- .2421.4 3.9 .54 9.5 .11` 21.3" 4.0 .69 5.8 .54 `21.1 1.7 .60 9. s 15 20.163.5. .53' 9.1 .19 '20.6 3.5` 66 6. 1 63. 21.4 1. 6I .71 6.2 .61 21.2 v1.6 k .74 6.3 .67 '21.4 2 3' Reference is directed 4to Table III whichcontains a comparison of the stress-rupture properties exhibitedfhy. oneof the presently used commerciall alloys referred to as Alloy X and thealloy of the present invention referred to as Alloy B.

TABLE III Nominal Stress Rupture Properties [At temperature 1350 F.]

Hours Percent .Percent Alloy Stress to "Elonga-- Bed. of

' Failure tion Area- 20; 000 175 17.7' 29. 9V 20,000 '249 12.01 2.1.0'20,' 000 307 1(i."5A 18.9 20,000" 617' 28:0 34.0 20, 000. 647 29.0 37.0

It will be noted from the test results tabulatedk in. Table II-I thatthe alloy of. @the present invention exhibits outstanding'charaoteristics .which lare far superior to therme chanioal propertiesexhibited by Alloy X. Alloy Bierchibits greater rupture life and-is moreductile,.asxnteasured` by both the percent elongation and percentreduction .off area, than Alloy X. Since the test temperature vis therange of operation of exhaust valves ofintemal l.come bastion engines,:the foregoingtest results'. give la realistic appraisal of the use ofxthe alloy for valves and valve components.

yIn :addition to the foregoing. stress rupture properties, the Ialloy ofthe present invention must :also possesssutlcien-t hardness. In thisrespect, it =is customary. fertile valve manufacturer to specify aminimum. 'hardness' off about 34 Rc at room temperature, land-aminirrrumchardness of about 135 BH-Nat 1400 F. Reference .is directedto Table IV which includes a ltabulation .of the hardness properties-and the tensile properties offsomeofftheheats referred -to in 'FableII.

By comparison of-the heats listed-inTab1eI,.especially with respect tothe nickel content of said-sheatsait is clear that nickel contents `inexcessief abont1:9.0%- adversely affect the hardness andductizltyr ofvthe alloy.

Comparing Heat No. 40839 .with Heat No. 8, it is clear that the roomtemperature hardness is adversely affected. While Heat No. 8 has a yieldstrength comparable to Heat No. 40837 and a slightly lower tensilestrength than Heat No. 40839, it is clear that the ductility of thealloy is seriously affected. Thus it is clear that the nickel con- 'tentof the present Ialloy must be limited to a nickel content between 1.40%and 1.90% in order to obtain the requisite hardness and ductilitywithout adversely affecting the stability and machinability of thealloy.

Heretofore it has been stated that the alloy of the present invention isparticularly suitable for use las valves and valve components, andthrough various heat treatments and/or fabrication procedures it ispossible to obtain a wide variety of mechanical properties. Reference isdi- 'rected to Table V which shows the results of hardness measurementsconducted on Heat No. 05055, the chemical analysis of which is set forthin Table II, for aging treatments 'at various temperatures and forvarious periods of time, the alloy being in the hot worked or in the hotworked plus solution heat treated condition.

TABLE V [Heat No. 05055] From the `data tabulated in Table V, it isclear that the alloy of the present invention responds to a simple agingtreatment after the alloy has been hot worked. As thus processed, it isclear from the data set forth in Table V that an aging heat treatment ata temperature within the range between 1400 F. and 1700 F. for variousperiods of time of up to eight hours is effective for obtaining at least38 Rc hardness in the alloy. This outstanding hardness is obtained,together with ladequate creep rupture properties land without anysacrifice in the corrosion and oxidation resistance nor in themachinability of the alloy. The test results recorded in Table V alsoillustrate that the alloy of the presen-t invention develops a minimumhardness of Rc 34 where the alloy after hot working is subjected to asolution heat treatment at a temperature of 2150 F. followed by aging ata temperature within the range between 1400 F. and 1700 F. for varioustime periods of up to about eight hours. As thus processed, the alloyalso possesses excellent creep rupture properties Iand the corrosion andoxidation resistance and machinability are not impaired. Thus the alloyof the present invention is ideally suited for use in valves and valvecomponents.

As was stated hereinbefore, the alloy of the present invention is anlage-hardenable, stable, austenitic alloy which contains a criticalbalance of the chemical composition, and especially the nitrogen andsulfur contents. While the excellent hot workability exhibited by thealloy of the present invention is a significant factor in the overallcost reduction resulting from lower scrap losses and higher yields infabricating the material from the ingot form to the semi-finished millproduct, the most significant cost factor is realized from the excellentmachinability of the alloy having a composition within the specificanalysis range. These cost reductions are realized from the superiormachining -qua-lities of the alloy of the present invention resultingfrom increased production rates, less down-time for tool change, and asignificantly `longer tool life. Based on comparative machinlabilitytests con- 8 ducted under strict laboratory control, the machinabilityof these alloys was `determined to be critically dependent upon thebalance of the austenite and ferrite-forming elements, the sulfurcontent and the nitrogen content, all of which are interrelated. Sincethe chemical analysis of the material has an extremely profound eifectupon its machinability, the presence of the a'lloying elements withinthe range set forth hereinbefore in Table I is further characterized byreason of the fact that a critical balance must be maintained betweenthe elements which promote ferrite and the elements which promoteaustenite. By assigning a weighted `factor corresponding to eachelements relative strength in this respect, and by incorporating thesefactors into .a ratio of austenite-forming elements to ferrite-formingelements, a numerical index is obtained which represents the chemistryof each heat and the cornposition limits -within which each heat mustfall in order to obtain the :optimum combination of good hot workabilityand excel-lent machinability. This numerical index, then, must fallwithin certain given limits, as will be more clearly set forthhereinafter. Based on production experience, it has been found that theratio of the austenite formers to ferrite formers can be expressed as:

Using the foregoing equation, it has been found that from the aspect 'oftotal chemistry alone, `an alloy having a numerical index, as computedby the above formula, which falls within the range between 1.83 and 2.67will possess an outstanding combination of hot workability andmachinability. However, even though an alloy may have a numerical indexwithin the given range, certain other considerations are alsosignificant.

It has been found that the addition of sulfur to the alloy enhances itsmachinability. However, the range must be critically restricted inlorder to prevent hot workability diihculties from occurring to thealloy. Our experience has dictated that the sulfur must be presentwithin the range between 0.025% and 0.055% in order to effect theimprovement in machinability without causing any hot workingdifiiculties. Signicant improvement of machinability has not been foundin the subject alloy where the sulfur content is increased to above0.055%.

Accordingly, there is no advantage which exists to warrant running therisk of hot workfability difficulties with the use of higher sulfurcontents.

In addition to the foregoing, it has also been found that the nitrogencontent is also highly critical within the range stated in Table I.Nitrogen, being an interstitial hardener, has a great influence on thestrain hardening rate of the alloy. Moreover, the nitrogen content alsoprovides additional strength and materially contributes to theaustenitic stability of the alloy. The nitrogen content must be abovel0.18% in order to maintain the requisite degree of strength andhardness, together with suiicient austenitic stability, whereas if thenitrogen content is increased to above about 0.28%, the alloystrainhardens quite rapidly with the result that the machinability ofthe alloy is adversely affected.

Each of the critical factors, that is, the austenite to ferrite balanceas expressed by the numerical index, the sulfur content and the nitrogencontent, are quite interrelated. Consequently, it is necessary to/perform some test in which all the variables are eliminated, except thedifferences in the composition of the material tested. The subject test,which meets these conditions and which has been found to be mostpractical in evaluating the comparative machinability of these alloys,is a single point lathe turning test. This test was performed on a 16 x30 American Pacemaker lathe equipped with a variable speed drive toprovide constant speed control. A single point C-6 carbide tool was usedto machine l-inch diameter hot rolled bar stock in both the solutiontreated and the solution treated plus aged condition, A Constant speed 9of 100 feet per minute, feed at the rate of 0.009 inch per revolution,and depth of cut of 0.050 inch were maintainedfor all tests. The toolangles 4were also maintained constant-as follows:

Back rake-5 negative Side rake-5 negative Side cutting edge angle, 15End cutting edge angle, 15 Relief, 5

Nose radius, 1/32" The tests were run without a cutting fluid, and therelativemachinability of the alloy from each heat was ascertainedthrough -a measurement of the time required to produce a predeterminedamount of tool wear. Thus the combined effects of the aforementionedfactors of the ratio of the austenite formers to ferrite foriners asexpressed by the numerical index, the sulfur content and thenitrogencontent, can be illustrated by correlation with the relativemachinability ratings. These ratings are graphically illustrated by theplots of tool life in minutes versus the wear of the tool land inthousandths of an inch. Incomparing the curves set forth in FIGS. 1 and2, the optimum machining curve possesses a high slope and is located atthe left-most side of the plot. Thus-the machinability of the alloydiminishes as the curve shifts from left to right on the plot, and asthe slope decreases. As illustrated in FIG. l, Heat No. 40838 exhibitsthe best machinability vof all the heats reported since it hasn thehighest slope and is located at the left-most side of the plot; It issignificant to point `outV that the gamma to alpha ratio was ideal,being 2.29, the nitrogen content was at the high end of the analysisrange, andthe sulfur content was at about the high end of the range. Bythus maintaining the nitrogen and sulfur contents within the rangesspecified with the proper numerical index, the alloy exhibitsexeellentmachinability.

While Heat No. 04732 appeared to be the next most machinableheat, it wasfar inferior to Heat No. 40838. Comparison of the chemical analysesreveals that while both heats possessed similar sulfur contents,theextremely high nitrogen content of Heat No. 04732, together with itsextremely high numerical index (3.25), clearly illustrates the adverse`effect of these latter `two factors upon the machinability of thealloy.

`Continuing the comparison, Heat No. 40839, which exhibits a goodinitial machinability, clearly illustrates the adverse effect of highnitrogen contents on the machinability ofthe alloy. Similarly, Heat No.40802, which has a goodA numerical index and a nitrogen content withinthe given range, clearly shows the adverse effect of low sulfurcontents. Heat No. 40836 has a high numerical index and a low sulfurcontent, and consequently exhibits poor machinability.

Reference is now respectfully directed to FIG. 2 which illustrates therelative machinability of some of these steels when they are machined intheir solution treated plusagecl condition. Once again it is clear thatHeat No. 40838 clearly possesses outstanding machinabilitycharacteristics when compared with Heat No. 04588, the latter being ofthe Alloy X analysis. Heat No. 40802 is included for comparisonpurposes, and clearly illustrates the effect of the low sulfur contentwhere the optimum numerical index and nitrogen contents are present.

From the foregoing it is clear that the alloy of the present inventionmust be maintained within critical limits of the chemical composition asset forth in Table I. In addition, the chemical composition must be sobalanced `as to provide the proper austenite to ferrite-forming elementsratio as expressed by the numerical index. In addition, it has beenfound expedient to maintain a critically narrow range of sulfur,together with a narrow range of nitrogen, in order to maintain theoptimum combination of properties. As thus balanced, the alloy of thepresent invention exhibits excellent hot workabi-lity and 10machinability, and an acceptable combination of chemical, mechanical andphysical properties equivalent to the alloys which are presently beingused. By thus maintaining the proper balance of the foregoing factors,there appear to be no special metallurgical-skills, Yprocesses orequipment which are necessary in order to practice the subjectinvention.

We claim:

1. An austenitic alloy suitable for use at elevated temperatures inatmospheres containing combustion products of leaded fuels and having acomposition within the limits 0.65% and 0.75% carbon, 5.50% and 6.90%manganese, 0.45% andi0.85% silicon, 20.50% and122.0.% chromium, 1.40%and 1.90% nickel, 0.18% and 0.28% nitrogen, 0.025% and 0.055% sulfur andthe balance essentially iron with incidental impurities and which ischaracterized by possessing excellent machinability and hot workability.

2. An austenitic alloy suitable for use at elevated temperatures Ainatmospheres containing combustion products of leadedfuels andhaving acomposition within the;limits 0.68% and 0.73% carbon, 6.00% and 6.50%manganese, 0.60% and 0.70% silicon, 21.00% and 21:50% chromium, 1.60%and 1.75% nickel, 0.20% Vand 0.25% nitrogen, 0.025% and 0.040% sulfurand thebalance essentially iron with incidental impurities, and which ischaracterized by possessing excellent machinability and hot workability.

3. An austenitic alloy suitable for use at elevated temperaturesinatmospheres containing .combustion products of leaded fuels and havinga composition containingabout 0.70% carbon, about 6.25% manganese, about0.65% silicon, about 21.25% chromium, about 0.7% nickel, about 0.22%nitrogen, 0.035% sulfur and the balance essentially iron with incidentalimpurities, and which is characterized by possessing excellentmachinability and hot workability.

4. An austenitic alloy suitable for use at elevated temperatures and inatmospheres containing combustion products of leaded fuels and which ischaracterized 'by exhibiting excellent hot workability andmachinability, said alloy having a composition within the limits of0.65% and 0.75% carbon, 5.50% and 6.90% manganese, 0.45% and 0.85%silicon, 20.50% and 22.0% chromium, 1.40% and 1.90% nickel, 0.18% and0.28% nitrogen, 0.025 and 0.055% sulfur and the :balance essentiallyiron with incidental impurities, andin which the austeniteformingcomponents and the ferrite-forming components of said alloy are balancedwithin the relationship:

40(% C+% N)+3(% N)+2(% Mr1l= Cr)+5.2(% Si) a to provide fy/a indexwithin the range between 1.83 and 2.67.

5. An austenitic alloy suitable for use at elevated temperatures and inatmospheres containing combustion products of leaded fuels and which ischaracterized by exhibiting excellent hot workability and machinability,said alloy having a composition within trhe limits of 0.68% and 0.73%carbon, 6.00% and.6.-50% manganese, 0.60% and 0.70% silicon, 21.0% and21.5% chromium, 1.60% and 1.75% nickel, 0.20% and 0.25% nitrogen, 0.025%and V0.040% sulfur and the Ibalance essentially iron with incidentalimpurities, and in Which the austeniteforming components and theferrite-forming components of said alloy are balanced within therelationship:

to provide 'y/x index within the range between 2.08 and 2.38.

6. Stable austenitic valves and valve components characterized by .goodresistance to thermal fatigue, hot stretching, thermal shock andcorrosion in atmospheres containing combustion products of leaded fuels,and

l 1 formed from an alloy consisting .essentially of 0.65 to 0.75%carbon, 5.50% to 6.90% manganese, 0.45% to 0.85% silicon, 20.50% to22.00% chromium, 1.40% to 1.90% nickel, 0.18%' to `0.28% nitrogen,0.025% to 0.055% sulfur and the .balance essentially iron withincidental impurities.

7. Stable austenitic Valves and valve components characterized by `goodresistance to thermal fatigue, bot stretching, thermal shock andconrosion in atmospheres containing combustion pnoducts of leaded fuels,and `formed from an alloy consisting essentially of 0.68% to 0.73%carbon, 6.00% to 6.50% manganese, 0.60% to 0.70% silicon, 21.00% to21.50% chromium, 1.60% to 1.75% nickel, 0.20% to 0.25% nitrogen, 0.025%to 0.040% sulfur and the balance essentially iron with incidentalimpurities.

8. An article :of manufacture of the class including valves and valvecomponents which are suitable vfor use in exhaust systems of internalcombustion engines, said article exhibiting good strength and hardnessat operational temperatuires of about 1400 F. and good resistance tocorrosion, oxidation, thermal fatigue, lliot stretchin-g and thermalshock, said article being formed from a stable, austenitic,age-hardenable alloy which is characterized by exhibiting excellent hotworkability and machinability resulting lfrom balancing the compositionwithin the range between 0.65% and 0.75% carbon, 5.50% and 6.90%manganese, 0.45% and 0.85% silicon, 20.50% and 22.00% chromium, 1.40%and 1.90% nickel, 0.18% and 0.28% nitrogen, 0.025% and 0.050% srulur andthe balance essentially iron with incidental impurities, to provide anumerical index within the ran-ge between 1.83 and 2.67 of theaustenite-forming components ('y) to ferrite-forming components (a) asexpressed by the relationship 9. An article of manufacture of the classincluding valves and valve components which are suitable for use inexhaust systems lof internal combustion engines, said article exhibitinggood strength and hardness at operational temperatures `of about 1400 F.and good resistance to corrosion, oxidation, thermal fatigue, hotstretching and thermal shock, said article being formed from a stable,austenitic, age-hardenable alloy which is characterized by exhibitingexcellent hot workability and machinability resulting from balancing thecomposition within the range between 0.68% and 0.73% carbon, 6.00% and6.50% manganese, 0.60% and 0.70% silicon, 21.00% and 21.50% chromium,1.60% and 1.75% nickel, 0.20% and 0.25% nitrogen, 0.025% and 0.040%sulfur and the balance essentially iron with incidental impurities, toprovide a numerical index within the range between 2.08 and 2.38 of theaustenite-forming components (fy) to ferrite-forming components (or) asexpressed by the relationship 10. Valves and valve components formedfrom a stable, austenitic alloy which possesses a composition within thelimits between 0.65% and 0.75% carbon, 5.50% and 6.90% manganese, 0.45%and 0.85% silicon, 20.50% and 22.00% chromium, 1.40% and 1.90% nickel,0.18% and 0.28% nitro-gen, 0.025% and 0.055% sulfur and the balanceessentially iron with incidental impurities, and which is characterizedby exhibiting a minimum handness of 38 RC after forming the valves orvalve components by `hot working vfollowed by an aging heat treatment ata temperature within the range between 1400 F. and 1700 F. for a timeperiod of up to about 8 hours.

11. Valves and valve components lformed from a stable, austenitic alloyhaving a composition within the limits between 0.65% and 0.75% canbon,5.50% and 6.90% manganese, 0.45% and 0.85 silicon, 20.50% and 22.00%chromium, 1.40% and 1.90% nickel, 0.18% and 0.28% nitrogen, 0.025%n and0.055% sulfur and the balance essentially iron with incidentalimpurities, and which is characterized by exhibiting a minimum hardnessof 34 Rc after forming the valves or valve components by hot workingfollowed by a solution heat treatment of the valves and valve componentsat a temperature within the range between 2100D F. and 2200 F. for atime period of brom 15 minutes to about 4 hours, quenching andthereafter aging at a temperature within the range between 1400 F. andl700 F. for a time period ranging between 4 hours and 24 hours.

12. Valve and valve components formed from a stable austenitic alloywhich possesses a composition within the limits between 0.65% and 0.75%carbon, 5.50% and 6.90% manganese, 0.45 and 0.85 silicon, 20.50% and22.00% chromium, 1.40% and 1.90% nickel, 0.18% and 0.28% nitrogen,0.025% and 0.055% sulfur and the balance essentially iron withincidental impurities, and which is characterized by .good creep ruptureproperties, and rhai'dness without any adverse effect on themachinability after forming the valves 'by bot working followed by asolution =l1eat treatment at a temperatune within the range between 1950F. and 2050 F. |ior a time period of up to one hour followed by aquench.

13. Valve and valve components formed finom a stable austenitic alloywhich possesses a composition Within the limits between 0.65 and 0.75carbon, 5.50% and 6.90% manganese, 0.45% and 0.85% silicon, 20.50% and22.00% chromium, 1.40% and 1.90% nickel, 0.18% and l0.28% nitrogen,0.025 and 0.055 sulfur and the balance essentially iron with incidentalimpurities, and which is characterized by being suitable for use in aninternal combustion engine without prior heat treatment after formingthe valves by hot working followed by a machining operation wherenecessary.

References Cited in the le of this patent UNITED STATES PATENTS`2,380,821 Breeler et al. e- July 31, 1945 FOREIGN PATENTS 481,629Canada Mar. 11, 1952

2. AN AUSTENITIC ALLOY SUITABLE FOR USE AT ELEVATED TEMPERATURES INATMOSPHERES CONTAINING COMBUSTION PRODDUCTS OF LEADED FUELS AND HAVING ACOMPOSITION WITHIN THE LIMITS 0.65% AND 0.75% CARBON, 5.50% AND 6.90%MANGANESE, 0.45% AND 0.85% SILICON, 20.50% AND 22.0% CHROMIUM, 1.40% AND1.90% NICKEL, 0.18% AND 0.28% NITROGEN, 0.025% AND 0.055% SULFUR AND THEBALANCE ESSENTIALLY IRON WITH INCIDENTAL IMPURITIES AND WHICH ISCHARACTERIZED BY POSSESSING EXCELLENT MACHINABILITY AND HOT WORKABILITY.